1. Field of the Invention
This invention relates to electronic circuitry, and in particular to a passive transponder, which when suitably activated by a transmitter, transmits an identification code and information pertaining to an analog parameter sensed by the transponder, and one or more digital parameters.
2. Prior Art
Numerous techniques are known in the art for detecting, counting, or identifying objects from a distance. One well-known class of such prior art devices relies upon optical detection of the objects being connected or identified. Well-known examples of such optical systems are the bar codes found on may household products which codes may be read by various automated techniques. Optical identification systems, however, require specified lighting conditions, and are subject to errors when the coded portion is not maintained in a clean condition.
Other techniques for counting or identifying objects at a distance utilize radio signals, however, many of these systems require an independent power supply for each object, thereby necessitating replacement of batteries or severe limitations on the type of objects which may be counted or identified. Also known in the art are solid state oscillators which are powered by incident radio frequency energy. Typically such devices have been expensive requiring high levels of transmitted power to drive the passive transponder. Other such devices have required various types of semiconductor memory for storing identification codes or other information, thereby making such transponders undesirably expensive or difficult to program in the field. Such transponders have also usually been of undesirably large size.
The present invention overcomes these disadvantages and provides an inexpensive small passive transponder operable at extremely low levels of incident radio frequency energy, and able to sense and relay desired analog and digital information concerning the environment in which it is situated, together with an identification code. The transponder of this invention may be simply programmed with an identification code in the field.
Typical prior art transponders are disclosed in U.S. Pat. No. 4,068,232 to Meyers and U.S. Pat. No. 4,075,632 to Baldwin et al.